COPPER MIGRATION IN CDTE HETEROJUNCTION SOLAR-CELLS

CdTe solar cells were fabricated by depositing a Au/Cu contact with Cu thickness in the range of 50 to 150 Angstrom on polycrystalline CdTe/ CdS/SnO2/glass structures. The increase in Cu thickness improves ohmic contact and reduces series resistance (R(s)), but the excess Cu tends to diffuse into CdTe and lower shunt resistance (R(sh)) and cell perf ormance. Light I-V and secondary ion mass spectroscopy (SIMS) measurem ents were performed to understand the correlations between the Cu cont act thickness, the extent of Cu incorporation in the CdTe cells, and i ts impact on the cell performance. The CdTe/CdS/SnO2/glass, CdTe/CdS/G aAs, and CdTe/GaAs structures were prepared in an attempt to achieve C dTe films with different degrees of crystallinity and grain size. A la rge grain polycrystalline CdTe thin film solar cell was obtained for t he first time by selective etching the GaAs substrate coupled with the film transfer onto a glass substrate. SIMS measurement showed that po or crystallinity and smaller grain size of the CdTe film promotes Cu d iffusion and decreases the cell performance. Therefore, grain boundari es are the main conduits for Cu migration and larger CdTe grain size o r alternate method of contact formation can mitigate the adverse effec t of Cu and improve the cell performance.